A standard die-casting apparatus has a pair of mold parts together defining a mold cavity and a feed sleeve set in one of the parts and defining a passage opening into the cavity. Molten metal is fed through the passage to the cavity.
Normally the sleeve feed passage is basically frusto-conical, flared along its full length toward the passage. To hold back the molten metal when the mold is opened, the small-diameter outer end, right at the injector nozzle of the metal-supplying machine, is cooled so as to form a small plug. When the workpiece has solidified and is knocked out of the mold cavity, the hardened metal in the passage therefore separates at this cooled region, leaving the workpiece carrying a long outwardly tapering sprue corresponding in shape to this passage. This sprue must be removed, typically with a separate grinding operation that is fairly laborious as the sprue is rather large. The metal of this sprue is frequently contaminated by the grinding operation so it cannot be recycled. Furthermore the mold cycling time is increased by the amount of time it takes this extra mass of metal to harden, and of course heat energy is wasted melting the metal of this sprue which is not used.